Nowadays, necessity has been voiced more and more for improvements in accuracy of the surface roughness measuring instrument wherein a fine displacement of a contact element brought into contact with the surface of a work to be measured is detected as an electric signal and this electric signal is processed in a predetermined manner to measure a surface roughness of the work.
With the measuring instrument of the type described, as a method of processing the electric signal thus detected, an analogue type method for directly integrating an analogue output signal from a displacement signal has heretofore been widely utilized. However, along with the progress in the electronic techniques in recent years, a digital type method of analogue-digitally converting the analogue output signal into a digital signal and digitally processing the same has been proposed.
When the digital type method is adopted, in general, the number of samplings of the measured data relating to the measuring accuracy poses a problem. When a predetermined length is measured for example, if the number of samplings is small, the intervals between the samplings become longer accordingly, thus lowering the measuring accuracy. In consequence, when the number of samplings is increased, a satisfactory measuring accuracy can be obtained. However, an increased number of samplings requires a high class measuring instrument of a high response speed type for example, so that economical restriction is imposed on this case.
Such being the case, to relieve the burden on the electric circuit, there has been proposed a method of such a type that, in accordance with the measuring length (L) of the work, a speed of a stylus relative thereto is changed with the speed of processing the sampling being made constant. However, by this method, the number of samplings per unit length is decreased with the increase of the measuring length (L), thus presenting the disadvantages that the larger the measuring length is, the lower the measuring accuracy becomes, and moreover, it is extremely difficult to strictly change the speed of the stylus relative to the measuring length (L).
Therefore, the present invention has been developed to obviate the above-described disadvantages of the prior art and has as its object the provision of a surface roughness measuring instrument being excellent as viewed from the standpoint of accuracy and economics, and high in practicability.